1. Field of the Invention
This invention relates to systems and processes for controlling the heating of a product, such as cooking foodstuffs, in a dielectric oven. Further, it relates to systems and processes for controlling dielectric ovens having multiple support levels for heating commercial quantities of the product. Particularly, it relates to systems and processes for controlling the current flow within capacitor plates which produce an electromagnetic field providing electromagnetic energy to the product.
2. Description of the Related Art
Commercial ovens are commonly convection ovens utilizing a slow convection heating process to heat products. Dielectric ovens, however, heat a product due to the electric, i.e., dielectric, losses caused when the product is placed in a varying electromagnetic field. If the product is homogeneous and the electromagnetic field is uniform, heat may develop uniformly and simultaneously throughout the mass of the product.
Dielectric ovens are known, and examples of such ovens are disclosed in U.S. Pat. No. 4,812,609 to Butot; U.S. Pat. No. 4,978,826 to DeRuiter et al.; and U.S. Pat. No. 4,980,530 to Butot, which are incorporated herein by reference. Such ovens may operate in a frequency range of 2 to 40 Mhz. Referring to FIG. 1a, a dielectric oven 200 may be fitted with guide racks 202 for stacking a plurality of trays 204 carrying a product 206 to be heated. These racks 202 also may function as electrodes for producing an electromagnetic field. A variable air capacitance 212 is created between tray 204 containing product 206 and electrodes 202 to control the electromagnetic energy applied to product 206.
Dielectric ovens may utilize an oscillating circuit or circuits having specially designed electromagnetic energy sources, such as power tubes. Such energy sources may be coupled and supply current to guide rack electrodes 202 via contacts 205 which project through an oven housing 209 into heating cavity 208. The oscillating circuit(s) generally provide a substantially fixed distribution of voltage and power within a heating cavity. Thus, longer heating times may be required for heating greater quantities of products. Further, frequencies at which the ovens are operated are dependent on the characteristics of the product being heated.
Referring to FIG. 1b, although dielectric ovens 200' may handle a plurality of vertically stacked trays 204', which permit products 206' to be heated at multiple levels 210' within a single heating cavity 208', only a single pair of electrodes 202' may be provided to apply the electromagnetic energy for heating. Thus, when a number of different heating levels 210' are used, the amount of energy applied to product 206' in each tray 204' may be reduced, and heating may take longer. As discussed above, electromagnetic energy sources may be coupled and supply current to electrodes 202' via contacts 205' which project through an oven housing 209' into a heating cavity 208'. A variable air capacitance 212' may be created between tray 204' (and product 206') and electrodes 202' to control the energy applied to product 206'.
A dielectric oven may include a heating cavity for receiving a tray containing the product, an electromagnetic energy source; oscillating circuit for producing an electric signal, and an electrode configuration for producing an electromagnetic field in the cavity to apply energy from the oscillating circuit to the product. Such ovens are broadly operable for increasing the energy applied from the oscillating circuit to the product, without increasing the operating voltage of the electromagnetic energy source or the frequency of its operation. These ovens may include a plurality of oscillating circuits having substantially similar resonant frequencies.
The oscillating circuits may receive power from a power robe in order to establish respective oscillating signals. More particularly, at least first and second oscillating circuits may be provided, and the electrode configuration may include at least first and second electrodes, each of which is a component of one of the at least two oscillating circuits. The product may be bracketed between electrodes of a capacitor in the oscillating circuit. The oscillating circuit is arranged to provide a voltage across the capacitor which is twice the voltage across the power source, thus permitting doubling of distance between the electrodes of the capacitor without reducing the electromagnetic field strength and increasing of quantities of the product which may be heated between the capacitor electrodes.
Each of the oscillating circuits may also include an inductance and a capacitance. The capacitance includes a pair of capacitors respectively formed between two capacitor plates, i,e., the electrodes of the oscillating circuit, and another pair of plates, for example, wall portions of the heating cavity. The two electrodes of each oscillating circuit may be oriented to produce an open electromagnetic field between them. In this configuration, electrode pairs form a pair of interconnecting load capacitors between the electrodes of the oscillating circuits. The dielectric of the load capacitors includes the product placed between the electrodes of the capacitors, i.e., within the capacitance.
This configuration produces an open electromagnetic field between the electrodes of each of the pair of interconnecting (load) capacitors. The open electromagnetic field has a power intensity distribution determined by the dielectric characteristics of the product, while permitting the electromagnetic energy source to operate at a substantially constant power level. Further, the use of the load capacitors as connectors between the oscillators isolates the frequency of oscillation of the oscillating circuits from the effects of the dielectric characteristics of the product. Thus, both the power intensity and the frequency of the power transferring signals are maintained more nearly constant, with reduced variations caused by the dielectric characteristics of the product being heated.
A variable air capacitance or air "gap" may be included between the electrodes of the interconnecting capacitors and the product for controlling the energy applied to the product between the load capacitors. See FIGS. 1a and 1b. Nevertheless, such a variable air "gap" may interfere with the rapid insertion and removal of trays containing products for heating. Further, because of the speed with which products may be heated in a dielectric oven, manual control of the dielectric oven may be difficult and inefficient. Because heating may include the thawing and cooking of frozen foodstuffs, accurate control of the oven prevents uneven or inadequate heating.